Constantine is probably in his twenties in AD 306 when his father, Constantius, dies at York. For most of the past ten years the young man has been at the court of Diocletian. But he has joined his father's camp in the previous year, 305, after the promotion of Constantius to the post of Augustus in the west. Constantine is therefore known to the legions in Britain. In 306, contrary to orders from the centre of empire, they proclaim him Augustus.

Compromises are made, giving Constantine rule over Britain and Gaul. But as more claimants emerge, Constantine gambles on a bigger share of power. In 312 he marches south and invades Italy.

Battle of the Milvian Bridge: AD 312

A mysterious decision by Constantine in October 312 can be seen now as one of the great turning points of history. He is camped just north of Rome, about to do battle with his rival for control of the western empire. He decides that his men shall wear on their shields a Christian symbol - the monogram known as the Chi-Rho, formed from the first two Greek letters of the word Christ.

Constantine wins the battle of the Milvian Bridge. His rival dies fleeing back over the Tiber when a bridge of boats collapses. In Constantine's mind he has won this crucial engagement in alliance with the god of the Christians. The results are dramatic.

Formally acknowledged by the senate as the Augustus of the west, Constantine immediately takes steps to favour the persecuted Christians. He restores confiscated church property and offers public funds to churches in need. In AD 313 he arranges a meeting in Milan with Licinius, one of two claimants to the title of Augustus in the east. He persuades him to follow the same policy.

Later in that year Licinius defeats his rival in the east. He too now proclaims a policy of religious toleration, offering compensation to the Christians for the wrongs done to them.

Within a year or two of suffering severe persecution, the Christians suddenly find themselves a favoured group within the empire. They win tax concessions, and Roman basilicas are constructed for their use as churches. There are career advantages within Constantine's administration if one is a professing Christian. Conversions follow.

But Licinius is less fully committed to the cause. In 320 he reverts to a mild persecution, dismissing Christians from the army and the civil service. Constantine marches against him, in 324, and again the Christian banner is victorious. Licinius surrenders after a defeat near Byzantium. A year later he is executed on a charge of attempted rebellion.

A new Rome: AD 330

Constantine, now in firm command of the entire Roman empire (the first man for a long while to be in that position), is planning another initiative as significant as his adoption of Christianity. Immediately after the defeat of Licinius he sets about rebuilding Byzantium as a Christian capital city - one in which pagan sacrifice, the central rite of imperial Rome until this time, is specifically forbidden.

The city is ready by AD 330 for a ceremony of inauguration. Byzantium acquires two new names - New Rome and Constantinople, the city of Constantine. The Roman empire, within eighteen years of Constantine's first victory, has a new religion, a new centre of gravity and a significant change of culture.

Carthage and Donatus: AD 313-316

With the reckless enthusiasm of a convert, Constantine flings himself from the start into the various controversies dividing the Christian church.

The first issue, confronting him as soon as he wins power, has more to do with ecclesiastical politics than with heresy. The church in Carthage is squabbling over who shall be its bishop. A puritanical faction claims that the official nominee was a collaborator with the Roman authorities during the persecution of AD 303. They propose in his place a cleric by the name of Donatus, and in 313 they appeal to Constantine.

Constantine asks the pope in Rome and three bishops to look into the matter. They find against Donatus, whose followers appeal against the verdict. The emperor, wishing to appear open-minded, summons more than 300 bishops to Arles to consider the case. They too find against Donatus. In 316 the Donatists appeal directly to Constantine himself. He upholds the finding and orders that Donatist priests be removed from their churches.

Persecution provides martyrs and martyrs nourish sects; the Donatists will remain a thorn in the side of the north African church for the next three centuries. But Constantine has acquired a dislike of dissent. And soon he is confronted with a far more disruptive schism in Alexandria.

Alexandria and Arius: AD 323-325

The heresy associated with the name of Arius, a priest in Alexandria, is the most significant in the history of Christianity. It concerns the mystery at the very heart of the religion - the Trinity.

The problem for the early church has been that the Gospels talk of God and of Jesus (who describes God as his Father) and, more occasionally, of the Holy Spirit. But they do not explain how they relate to one another. All three seem to be divine, and yet - as a sect of monotheistic Judaism - early Christians know for sure that they worship only one God. How can this be? The concept of the Trinity, three in one and one in three, gradually emerges as the best answer. But it begs many questions.

Only if the three are equal can they be aspects of one god. Yet if God creates Jesus, he clearly has some sort of priority. On the other hand if God does not create Jesus, he can hardly be his Father. This is the problem which concerns Arius, who asks in particular whether there was ever a time when God existed but Jesus, as yet, did not. He concludes that there was such a time ('there was when he was not'). Jesus is therefore less than fully divine.

Even so, Arius agrees that it is right to worship Jesus. This reopens the door to polytheism, and in 323 the bishop of Alexandria dismisses his troublesome priest. The dispute rapidly escalates. In 325 Constantine intervenes, summoning a council at Nicaea.

Nicaea and orthodoxy: AD 325

More than 200 bishops, mainly from the eastern parts of the empire, arrive at Nicaea for the council. They meet in Constantine's palace, and the emperor himself presides over many of the discussions. His authority is purely political; though an undoubted supporter of Christianity, he has not yet been baptized.

The alarming presence of the emperor helps the bishops to reach a conclusion more emphatic than is justified by the range of their opinions. The crack opened wide by Arius seems to be firmly closed when it is announced at Nicaea that the Father and the Son are of the same substance (homo-ousios in Greek).

Good works and a late baptism: AD 325-337

The Arian heresy has not been suppressed as conclusively as Constantine may like to think, but his last years are calm enough for him to devote himself to his chosen religion. He becomes increasingly pious, and is a generous builder of churches in Rome and Palestine as well as Constantinople.

In Jerusalem he constructs the church of the Holy Sepulchre on the supposed site of the Crucifixion. Happily the excavations reveal what is taken to be the actual cross (or True Cross) on which Christ died. The discovery is later credited to the emperor's aged mother, Helena, who in 327 makes a famous pilgrimage to the Holy Land.

Falling ill in AD 337, Constantine is at last baptized - only a few days before his death. It has often been asked why he left this necessary act of Christian commitment so late. The answer is probably so as not to waste the magic of baptism, which washes away sins. An emperor can hardly live a blameless life, and there are many blots on Constantine's record - such as his unexplained execution of his eldest son and his second wife in 326.